For focusing on a particular sound (such as rr or ñ), tongue twisters can be a productive warm-up activity. This website provides a useful selection of Spanish tongue twisters.
1. Go through the twister one short phrase at a time, with the class repeating after you.
2. Go through again, putting together somewhat longer phrases.
3. If needed due to length, repeat step 2
4. Have the class recite the tongue twister together with you in its entirety.
5. Pick out sections of the class or brave individuals (or people who come in late) to recite the tongue twister solo.
With their simple vocabulary and limited vocal range (let's face it, your students won't all be singers), children's songs can be a good way to teach or reinforce some vocabulary and get the students thinking in the target language. One of my personal favorites for this is Los Pollitos Dicen (tener hambre, tener frío, the suffix -ito).
While the Three Stooges aren't exactly known for educational material, this song can be great for practicing tricky sounds and combinations of letters. I've had students tell me that this song gets stuck in their heads for days which, while potentially annoying, could also be quite helpful.
When the song is first introduced, use simple letters like B, D, K, etc. H is also a good one for early use to help students reinforce the idea that H is silent in Spanish.
Later, C and G can be used to discuss and practice the differences in pronunciation of the two depending on what vowel follows (C and G are hard before A, O and U but soft before E and I). This provides helpful scaffolding for when they later need to change between those vowels in verb conjugation (such as the formal command form: Jugar -> Juegue).
Another day, you can substitute dipthongs for the vowels to demonstrate their mono-syllabic nature.